This invention relates to an improved assembly for mounting a semiconductor element or device. In particular, this invention relates to an assembly which permits a semiconductor element to be used at higher power levels than previously possible by enhancing the cooling of the element through heat sinks which are maintained in intimate thermal contact with the semiconductor device during repeated thermal cycles.
As noted in our copending application, proper use of a semiconductor element requires maximum dissipation of heat generated in the semiconductor during use. This dissipation is particularly important in power semiconductors which may be defined as semiconductors handling an amp or more of current. Failure to properly dissipate heat generated in the use of such a semiconductor impairs the electrical characteristics of the semiconductor and also sets up internal mechanical stress on the semiconductor material interfaces which may ultimately affect the long-term operation of the device. Power semiconductors are normally mounted in an assembly of heat sinks that facilitate cooling. These assemblies ordinarily comprise large masses of conductive metal materials appropriately isolated from the semiconductor and with the heat sinks themselves connected to the ambient atmosphere or to other heat conductive members. Maximum heat conductivity has heretofore been achieved by providing heat sinks on both major surfaces of the semiconductor element. More recently, these heat sinks were secured in thermal contact to the major faces of the semiconductor device under substantial forces approximating 500 to 10,000 pounds per square inch. To achieve these pressures, the available heat sink assemblies have provided packages with specially designed clamps and springs intended to produce and maintain the required pressures. These structures are relatively bulky, complicated and expensive. In addition, these designs do not ordinarily provide electrically insulated power connections. This in turn requires special assemblies and utilization of the power packs that take into consideration this lack of electrical insulation. In addition, in the assemblies heretofore available, the compressive forces holding the heat sinks in intimate contact with the semiconductor device tend to diminish as the semiconductor device is used. This diminishing compressive force results from the multiple thermal cycles to which the semiconductor device is normally subjected. During each use, the operation of the semiconductor increases the temperature of the device and creates temperature differentials and gradients in and between the various elements. This temperature gradient coupled with differences in coefficient of expansion result in relative movement of the semiconductor device and the element forming the heat sink package. As a result of this movement, the assembly loosens with a consequent reduction in the compressive forces applied to the assembly.
This limitation in prior art devices includes those assemblies in which the semiconductor device and heat sinks are held in relation by bonding adhesives. Thus, for example, the assembly shown in our co-pending application will function for prolonged periods of time, but such an assembly does appear to have a tendency to lose its efficiency due to decreased thermal conductivity from the semiconductor device after prolonged use. Although the relative movement is small (measureable in microns) such movement nonetheless does decrease the efficiency of the assembly.
It is therefore an object of the present invention to provide an improved assembly for mounting a semiconductor device such that the power handling capability of the device is increased. It is also an object of the present invention to provide an improved means of dissipating heat from a semiconductor device in an assembly which does not tend to lose its thermal conductivity efficiency over extended periods of use.
A further object of the present invention is to provide a simple, compact and comparatively inexpensive means for mounting one or more power semiconductor devices in a thermally conductive package.
A further object of the present invention is to provide a heat sink assembly for a semiconductor device which does not require maintenance and which is rugged and capable of significant mechanical abuse.
A further object of the present invention is to provide an improved means of connecting heat sinks to both sides of a semiconductor device in an assembly which permits a single design of major components that can be embodied in different arrays of semiconductor devices.
A further object of the present invention is to provide a heat sink semiconductor assembly that permits simple bus bar interconnections of multiple semiconductor packages.
One more object of the present invention is to provide an improved semiconductor assembly utilizing an organic bonding material for securing the heat sink components to the semiconductor devices which simultaneously functions as an electrically insulating media for the semiconductor device.
A still further object of the present invention is to provide an improved heat sink assembly for packaging semiconductor devices under compression in such a fashion that increases in the temperature of the assembly will result in the application of greater compressive forces between the heat sink components and the semiconductor device, thereby increasing the efficiency of the device with increases in temperature.
One further object of the present invention is to provide an improved heat sink assembly in which the heat sinks in contact with the semiconductor device are designed and shaped also to function as self-actuating springs that increase the compressive force on the semiconductor device with increases in temperature.
Other objects and advantages of the present invention will become apparent from a consideration of the detailed description of the invention.